Theoretically, if the only problem is the sleep when watching movies, it should not be much of a board problem. Yes, the board might be wearing out, but it still might have many years left. You could try reinstalling OSX, with an Archive and Install, and see if the problem disappears. The archive and install will preserve all your settings, documents, etc.
The high cpu usage with flash should not cause the sleep problem. Adding RAM can help with flash. I buy RAM from OWcomputing.com, a Mac specialist. If you do buy it, make sure to get a matched pair for your machine.
Flashblock is really good and I also use it on Firefox. I've also found that if you reset Safari (Safari menu/Rest Safari) relatively often, you avoid a lost of crashes caused by flash content. I leave cookies and saved passwords unchecked when I reset.
Using OSX 10.3.9 & 10.4.11 on a G4, G5 & Intel Macbook